1. Field of the Invention
The present invention relates to a contactless IC card for receiving a carrier wave amplitude-modulated with a modulation factor lower than 100% and demodulating the amplitude-modulated carrier wave to recover data carried thereon, and in particular relates to a technique for preventing incorrect data recovery in the demodulation.
2. Description of Related Art
In recent years, a contactless IC card system made up of an IC card and a reader/writer (hereinafter, “R/W”) which performs contactless data communication with the IC card through a fixed-frequency carrier wave has been increasingly considered for adoption in systems such as train ticket collecting systems, security systems, and electronic cash systems. The general construction of such a contactless IC card system is briefly explained below.
FIG. 1 shows the general construction of a contactless IC card system. As illustrated, the contactless IC card system is roughly made up of a contactless IC card 10 equipped with an IC 11 and a loop coil 12, and an R/W 30 equipped with a loop coil 21, a modulation/demodulation unit 22, a control unit 23, and an input/output unit 24.
The loop coil 21 serves as an antenna for transmitting/receiving a carrier wave which has been modulated with data, to/from the contactless IC card 10. The modulation/demodulation unit 22 modulates a carrier wave with data to be transmitted to the contactless IC card 10, or demodulates a carrier wave received from the contactless IC card 10 to recover data piggybacked thereon. The control unit 23 exercises control over the entire R/W 30, including control of modulated/demodulated data. The input/output unit 24 performs data input/output.
The R/W 30, which is installed in a train ticket collecting gate or the like, performs amplitude-modulation (ASK (Amplitude Shift Keying) modulation) on a carrier wave of a predetermined frequency (e.g. 13.56 MHz) using data to be transmitted, and transmits the amplitude-modulated carrier wave to the contactless IC card 10 which is used for a season ticket or the like. Thus, for data transfer from the R/W 30 to the contactless IC card 10, ASK modulation is employed that defines digital data 0 and 1 in accordance with the level of the amplitude of the carrier wave. Here, the modulation factor of the ASK modulation never reaches 100%. The use of ASK modulation with a modulation factor below 100% enables high speed transfer with a narrow occupied bandwidth, and therefore allows a contactless IC card to obtain a proper demodulated signal.
The contactless IC card 10 is a contactless IC card that contains no batteries. In this contactless IC card 10, the loop coil 12 receives the amplitude-modulated carrier wave from the R/W 30. The IC 11 demodulates the received carrier wave to recover the original data carried thereon, according to the demodulation method that corresponds to the modulation method employed in the R/W 30. The IC 11 then performs a predetermined process on the recovered digital data. After this, the contactless IC card 10 transmits a response signal to the R/W 30.
As is clear from the above description, data processing in the contactless IC card 10 is mainly conducted by the IC 11. The construction of this IC 11 is explained below.
FIG. 2 is a block diagram showing the construction of the IC 11. Since the contactless IC card 10 has no batteries, it derives DC power by rectifying the carrier wave transmitted from the R/W 30.
The IC 11 includes a rectifier 40 connected with the loop coil 12 which serves as an antenna for transmitting/receiving a carrier wave to/from the R/W 30, a modulation/demodulation unit 41 connected with the rectifier 40, a control unit 42, a memory unit 43, and a voltage regulator circuit 44. Though the rectifier 40 and the modulation/demodulation unit 41 are connected in series in the figure, they may be connected in parallel.
Once the loop coil 12 has received the ASK-modulated carrier wave from the R/W 30, the rectifier 40 rectifies the carrier wave to generate a power supply voltage, and a demodulator circuit provided in the modulation/demodulation unit 41 demodulates the rectified carrier wave to obtain a demodulated signal.
Here, the demodulated signal contains not only data but also other information such as commands and addresses. The control unit 42 processes the demodulated signal based on these information, after which the data is stored in the memory unit 43. Here, the control by the control unit 42 is done in accordance with a clock signal generated from the carrier wave by a clock generator circuit (not shown in the figure).
The voltage regulator circuit 44 regulates the power supply voltage generated by the rectifier 40 not to exceed a certain threshold voltage. This voltage regulator circuit 44 is a so-called shunt regulator that can protect the circuits inside the contactless IC card 10 from getting damaged by overvoltage, in cases such as where the distance between the contactless IC card 10 and the R/W 30 becomes too short.
A typical construction and operation of the demodulator circuit provided in the modulation/demodulation unit 41 in the contactless IC card 10 are explained next. FIG. 3 is a circuit diagram showing an example construction of the demodulator circuit.
A voltage is generated at both ends of the loop coil 12 when the loop coil 12 receives an ASK-modulated carrier wave from the R/W 30, and the generated voltage is inputted in the demodulator circuit as a power supply voltage (hereinafter, “Vdd”) after undergoing rectification and envelope detection. Resistors 901 and 902 are coupled to the input of Vdd, and capacitors 903 and 904 are coupled to the junction (hereinafter, “node A”) of the resistors 901 and 902. The capacitor 903 is a smoothing capacitor for eliminating noise which remains after the rectification by the rectifier 40.
The terminal of the capacitor 904 on the opposite side of node A is connected to one input terminal (hereinafter, “node B”) of a comparator 908, with one end of node B being coupled with a resistor 905 that is connected to a reference voltage generator circuit for generating a reference voltage (hereinafter, “Vref”). The capacitor 904 and the resistor 905 constitute a differential circuit. Through this differential circuit, only high-frequency components of Vdd having been voltage-divided by the resistors 901 and 902 are conveyed from node A to node B.
The reference voltage generator circuit is also connected to the other input terminal (hereinafter, “node C”) of the comparator 908 through a resistor 906. The comparator 908 is equipped with a latch, and is constructed so as to invert its output (i.e. demodulated signal) when the input voltage of node B exceeds a certain level relative to the input voltage of node C. More specifically, the comparator 908 has a hysteresis characteristic between two threshold values (upper and lower threshold values with respect to Vref). With such a characteristic, it is possible to prevent the output of the comparator 908 from being inverted every time a slight change appears in power supply voltage waveform for some reason.
FIG. 4 is a timing chart showing the voltage level of each node in the demodulator circuit shown in FIG. 3. As illustrated, the power supply voltage (Vdd) generated from the ASK-modulated carrier wave received by the loop coil 12 is voltage-divided by the resistors 901 and 902, and the resultant voltage is developed at node A. Differential components of this voltage at node A are propagated to node B. If the voltage at node B exceeds any of the two threshold values (indicated by the upper and lower horizontal dotted lines in node B in the figure) with respect to the reference voltage (Vref) at node C, the demodulated signal is inverted.
In the contactless IC card 10, the control unit 42 and the memory unit 43 consume power during their operations. Here, since the contactless IC card 10 draws its power from radio waves, its source impedance is high. This being so, momentary power consumption causes a sharp drop in power supply voltage, thereby disturbing the power supply voltage waveform and inducing such noise as indicated by arrow A or C in FIG. 4. Meanwhile, this power supply voltage waveform also carries data and accompanying information which need to be recovered through demodulation. Therefore, if noise large enough to exceed any of the threshold values of the comparator 908 is induced by a disturbance in power supply voltage waveform, the output of the comparator 908 is erroneously inverted even when there is actually no change of a data value between 0 and 1. When this happens, the original data cannot be recovered correctly.
For instance, noise due to a voltage sag at point A causes incorrect judgement of data 1 as data 0 (from point A to point B), or a voltage increase due to a rebound at point C causes incorrect judgement of data 0 as data 1 (from point C to point D).